The present invention relates, in general, to the fabrication of vacuum microelectronic devices, and more particularly, to field emission tips fabricated to improve efficiency and/or operating characteristics. The present invention includes the fabrication of field emission tips having a reduced work function and the fabrication of tips having reduced current fluctuation, improved noise immunity, more stable operation and longer device lifetime. The emission tips of the present invention include closely spaced, aligned gate electrodes.
Field emission sources of electrons, and more particularly, electron sources utilizing a plurality of conically shaped controllable electron emitters arranged in arrays or patterns are well known in the art, for it has been well established that electron emission can be stimulated by an electric potential applied near a cathode which tapers to a fine point. Such field emitters can be broadly categorized by the type of material used for fabrication. One such category includes the use of semiconductor material such as silicon or germanium to construct arrays of such emitters, while another category encompasses the use of sharply pointed metallic field emitters which utilize individual needle-like protuberances deposited on an electrode.
Deposited metallic emitters suffer from at least two major disadvantages. First, the use of deposition techniques to form the pointed shapes limits the area over which uniform arrays can be formed, for such techniques require that a source of emitter material be directed onto a surface essentially normal to that surface while at the same time directing a source of masking material onto the same surface at a shallow grazing angle. This is a very critical operation which does not lend itself to the formation of large numbers of emitter elements over large surfaces, principally because it is extremely difficult to obtain uniformity in the emitters. It is important that each emitter element in an array have essentially the same electron emission characteristics if the emitter array is to produce satisfactory results. However, a 10% variation in the radius of an emitter tip can result in a 300% change in current emission from that tip, and accurate control of the tip radius is difficult to achieve with deposition techniques. A further problem is that the fabrication of such prior devices entails the use of thin film techniques which produce relatively delicate non-uniform structures that are sensitive to the strong electrical forces characteristic of field emission.
Emitter arrays with metal tips have been fabricated in several sizes, ranging from single tips to arrays of over 5.times.10.sup.3 tips, with packing densities of up to 1.5.times.10.sup.7 tips/cm.sup.2. But such tips often require a high temperature cleaning process which limits the metals that can be used and limits the fabrication process.
Semiconductor materials such as silicon have produced densely packed arrays of emitters having atomically sharp tips (with tip radius less than 10 nm). Although Gallium Arsenide has been used, single crystal silicon has been more common, and various Si field emitter configurations have been produced. The work function for single crystal silicon is comparable to that of metal, i.e., about 4 eV, so that the emission field strength required for each is about the same. However, field emission from Si tips requires elaborate cleaning schemes for cleaning the tip surface and stripping the thin layer of native oxide that occurs. Further, Si tips, due to their sharper tip diameters, are not capable of producing as large currents as metal tips, and are less resistant to irradiation.
A problem common to both categories of emitter is due to the fact that in order to control the emission of electrons from such emitter arrays, gate electrodes are needed above, below, or near the emitter elements. The gates allow appropriate voltages to be applied between the emitters, the gate electrodes and an anode located above the emitters and gates so that the flow of electrons from the emitters is controllable. To allow electron flow from the emitter tips to the anode or collector electrodes, holes typically are formed in a gate electrode metal layer above or around the emitters. The size and precise location of the holes, and the voltage applied to the gate electrode, control not only the magnitude of the electron emission from the emitter, but also determine the shape of the emitted electron flow pattern and can determine the direction of the electron beam emitted from the emitter array. The hole size and its proximity to the emitter determine the voltage required for control of the current from the emitter, while the alignment of the axis of the hole with respect to the axis of the emitter determines the direction of the current beam from the emitter. However, precise alignment and hole size control has been very difficult to achieve in the prior art because of the very small geometries and tolerances in the devices. Typically, in order to obtain precise alignment it has been necessary to employ a difficult and time-consuming masking step, but even slight errors in the mask have created serious problems. The difficulties encountered in fabricating such arrays increase significantly as the dimensions of the emitters and the emitter arrays are decreased to the submicrometer or nanometer scale. Various approaches to the fabrication of such devices are described, for example, in U.S. Pat. Nos. 3,789,471, 3,921,022, 4,095,133 and 4,940,916.
Conventional field emission cathodes usually operate at very large potentials, typically greater than 10 Kv, or operate at very high temperatures, often in excess of 500.degree. C., or both. These requirements make them unsuitable for many applications, particularly in microstructures which are very sensitive to both voltage and temperature.